1. Field of the Invention
This invention relates to sense amplifiers, and in particular to temperature compensated sense amplifiers employes in programmable read-only memories.
2. Description of the Prior Art
In a device such as a programmable read only memory (PROM), it is necessary to provide an amplifier stage, or sense amplifier, to detect voltage changes indicative of shifts in logic levels. The threshold level of the sense amplifier is set between the high and low logic levels, and the voltage difference between the threshold and low levels defines the noise margin. To maximize the operating speed of the device, it is desirable to reduce the noise margin as much as possible, so that the signal rise time from low level to threshold level is minimized. Unfortunately, the noise margin tends to change with temperature, because the low level and the threshold level have differing tracking rates with respect to temperature.
Generally speaking, the threshold level will tend to converge on the low logic level at high temperatures, and will diverge from the low logic level at low temperatures. As a result of these characteristics, the device will have a slower operating speed at lower temperatures. Due to the convergence at high temperatures, the noise margin must be made sufficiently large at high temperature to ensure operation of the device. Ideally, the threshold level and the low logic level should track over temperature at the same rate, providing a constant noise margin and a constant operating speed.
U.S. Pat. No. 4,215,282 discloses a PROM. In this device, controlled current sources are used to compensate for threshold shifts. A current mirror with the same type of current sink is connected to the sense amplifier. This scheme requires a column clamp, or a bit line clamp connected to the sense node. It compensates only for threshold variations, while other temperature effects such as gain changes in the transistors and parasitic capacitance changes are not compensated.